Some questions

is possibile to setup ispconfig on a server with no hosting services on it and then manage multiple hosting servers (10-12) from this single instance?
for example, we would like to make a VPS dedicated to control panel only and from this VPS manage 10 or 12 phisical servers with apache,dovecot and so on.

I have three virtual servers. ispconifig is installed on all three and all three offer web/mail/etc... services. Only on one server is the web interface installed and available. From that interface, I control all three servers.

Now my "control" VM also serves services, but that is optional: you decide on what servers clients run their stuff, so just never assign the control vm.

More questions:
1) is possibile to backup a single user account and then be able to restore it (maybe on another server) with every configuration? (like DirectAdmin does)
What happens if this user account has services on multiple server?

I'm trying to use a multiserver setup.
The main server works properly.
I've configured a second server, connected it to the master with success, but any domain added from the control panel to that server, is not configured.

Did you run this on the slave and not the master and did you enable debugging before you run the command? According to the output, debuffing was not enabled or the command was not run on the salve server.

Every time i try to add a new user, the whole panel will hang and I have to restart apache.

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About what kind of user are you talking? We have sys users, remote users, email users, ftp users, ssh users, site users, webdav users? And which guide did you follow to prepare the servers for the installation of ispconfig?

I've ran on the slave.

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Then the slave cant connect to the mysql db of the master. This happens e.g. when the hosts files on both servers are not prepared in the way that is described in the multiserver guides. You can verify that by using the master mysql login details from the ispconfig config.inc.php file to connect from the slave to the master on the commandline.

The root user is only required for the installation, it is not used by ispconfig when connecting to the master.

Is not possibile to create an unprivileged user?

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Thats what the ispconfig installer is doing, but without the correct entrys in the hosts files, these users are wrong now so that the salve cant connect to the master. To fix that, uninstall ispconfig on the slave, delete the record for the slave server in the server of the master table and then reinstall ispconfig on the slave.

Your wrong here, ispconfig never uses the root account (at least if you have not modified the ispconfig sources ) and the ispcsrv user has priviliges for the tables where it needs access to as it has no global privs of course, see all the other privilige tables in the mysql db, you will find them there. If the ispcsrv user cant access the master from the slave, then your hosts files or server hostnames are still not correct. The lines in the hosts files have to be exactly as described in the guides and they have to match with the fqdn of the server and they have to be fqdn only, so this is ok:

123.123.123.123 server.hosts.tld

while this will not work with mysql:

123.123.123.123 server.hosts.tld server

or

123.123.123.123 server

and the commands:

hostname

and

hostname -f

have to retutn both:

server.hosts.tld

if you configured the servers correctly and as described in the guides. If:

hostname returns "server" while hostname -f returns "server.hosts.tld" then your system is not installed properly.